Progress in modem technologies is based on creating new materials and developing optical elements with desired properties based thereupon. In particular, a necessary element in the design of modem displays is an optically anisotropic film possessing the optimum combination of characteristics for a given application.
A number of polymeric materials may be employed in the manufacture of optically anisotropic films. The anisotropic optical properties of these films result from uniaxial extension and modification with organic or inorganic (iodine) compounds. Poly-vinyl alcohol, (PVA) is commonly used as the base polymer as described in Liquid Crystals: Applications and Use, B. Bahadur (ed.), World Scientific, Singapore (1990), Vol. 1, p. 101–103. However, the low thermal stability of PVA-based films limits their applicability. New methods for the synthesis of optically anisotropic films possessing improved characteristics are needed for these reasons.
Organic dichroic dyes may be used for the synthesis of optically anisotropic films that exhibit excellent optical and workability characteristics. Films based on such compounds are obtained through application of a liquid-crystalline aqueous dye solution onto a substrate surface, followed by evaporation of the solvent, such as for instance water. Anisotropic properties may be imparted to the films either through preliminary mechanical orientation of the substrate surface, such as is described in U.S. Pat. No. 2,553,961, or by means of an external orienting action, such as for example mechanical, electromagnetic, or the like, exerted on the film material while it is in a liquid crystal state. This approach is explained in greater detail in PCT patent publication WO 94/28073.
Although the liquid-crystalline properties of dye solutions have been known for some time, extensive investigations of these systems have begun only recently. The new research efforts have been stimulated by the capability of some of these dyes of forming “chromonic” liquid crystal systems. A distinctive feature of chromonic systems is that dye molecules are packed into supramolecular complexes having the form of columns, which are the structural elements of a mesophase. The highly ordered structure of dye molecules in these columns allows use of these mesophases for forming strongly dichroic oriented films.
Molecular structures, phase diagrams, and the mechanisms of molecular aggregation in various chromonic systems, including organic dyes, have been previously reviewed (i.e. Lydon, J. Chromonics, in: Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998), Vol. 2B, pp. 981–1007). A special feature of dye molecules that form chromonic mesophases is the presence of peripheral groups that render these dyes water-soluble. The main structural unit of all chromonic mesophases is a column of stacked molecules. The chromonic mesophases of organic dyes are soluble, possess a special structure, and are characterized by specific phase diagrams and optical properties.
By using dichroic dyes capable of forming lyotropic liquid crystal (LLC) systems, it is possible to obtain films possessing a high degree of optical anisotropy. Use of high-strength dyes for the film formation, produces films characterized by high thermo- and photo-stability.
These properties have produced substantial interest in LLC systems for use in optical films. Development of new methods for creating the dye-based films through the optimization of deposition procedure have been investigated, as have new compositions of lyotropic liquid crystals (LLC). New LLC compositions may be developed through introduction of modifying additives, stabilizing additives, surfactants and other additives to the known dyes, thus improving film characteristics as described in RU 2047643 and WO 99/31535.
Anisotropic films that are selective in different wavelength ranges are required by a growing number of new products. It is therefore desirable to develop new varieties of compounds capable of forming an LLC phase and films with the required properties. Films with different absorbance maxima location in a wide spectral range from the infrared to the ultraviolet are also desirable. However, only a small number of currently available dyes are useful in the formation of lyotropic mesophases. Thus, each new LC dye is now an object of close attention.
Indanthrone disulfoderivatives are dichroic dyes capable of forming a stable lyotropic LC phase that are also useful for preparation of optical anisotropic films. Lyotropic mesophases in disulfoindanthrone—water systems have been investigated by V. A. Bykov etc. “Lyotropic mesophases in the system disulfoindanthrone—water,” Zhurnal Phyzicheskoi Khimii. Vol. LXIII, No. 3, “Nauka”, 1989, pp. 793–796. Indanthrone is a vat dye that is practically insoluble in water that may be converted to a water-soluble form through a sulfonation process. To produce the disulfoderivative, an effective amount of indanthrone and oleum is added to chlorosulfonic acid under prescribed conditions. When the reaction completed, the mixture is cooled and diluted with water. The resultant precipitate is filtered, washed with hydrochloric acid, and dried. The produced disulfoindanthrone is then dissolved in water and purified. Based on analysis of the disulfoindanthrone—water system properties, it appears that a stable lyotropic hexagonal mesophase is formed starting from a certain dye concentration in a certain temperature range. The isotropic phase, the two-phase transition regions and the nematic phase are observed in rather narrow temperature and concentration ranges. The existence ranges for chromonic-type nematic and hexagonal phases in a disulfoindanthrone—water system have been previously investigated and reported in N. M. Kormilitsin, N. V. Usol'tseva, V. V. Bykova, G. A. Anan'eva, “Supramolecular Organization in the Disulfoindanthrone-Water System” (1999). Kolloid. J., 61, 75 (in Russian). On the basis of X-ray diffraction analysis data and stereochemical relations, a model of disulfoindanthrone supramolecular aggregates packing has been suggested.
Various dyes based on indanthrone disulfoderivatives for use in polarizer film fabrication have been previously described. Among them is the dye 3-chloroindanthrone-4,4-disulfoacid of the structure: which is selective in the wavelength range of approximately 620–680 nm as described in RU 1753700. Mixtures of LC disulfoindanthrone have been described in RU 2047643 (EP 961138), as have different modifying additives for improving characteristics of anisotropic films based on these compounds. Various disulfoindanthrone derivatives with different substituents have been disclosed in PCT publication WO 94/28073, as have disulfoindanthrone derivatives with different organic cations in a structural formula in PCT publication WO 99/31535.
One of the main disadvantages of previously described water-soluble indanthrone disulfoderivatives is the complexity of producing anisotropic films with uniform properties over the substrate surface based on these compounds. The complexity results from the high viscosity of LC solutions, their phase instability, and the likelihood of disorientation zones and micro-defects being formed by insufficiently uniform micro- and macro-crystallization proceeding during solvent removal after the liquid crystal is coated on a substrate surface. The high viscosity of the solutions based on currently available dyes also increases the probability of films of differing thickness being produced. This further decreases the reproducibility of the resultant film parameters. The viscosity of an LC system may be lowered by decreasing the dye concentration in the solution, but this may inhibit production of films with desired optical transmittance. These drawbacks complicate the process of forming films with high optical characteristics. Poor reproducibility necessitates accurate adjusting and strict control of fixed technological conditions at each film forming stage from coating to drying which may dramatically increase film production expenses.